maximum frequency suggestion 2

[pippogarruz]

User's group

I m working on a centrifugal fan wheel and I'm studing the natural
frequency of the rotor. This rotor has a shaft with two bearing at the
ends. All the pieces are steel made. The first shape is at 19Hz.
What is your opinion about the maximum velocity to avoid resonance
problem?

Thank you
Filippo Garuti
Italy

 

[electricpete]

As a general rule of thumb, operating rotational speed should be about 20% away from a resonance so 20% less than 19hz. Somewhere around 15hz (900 rpm).

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[GregLocock]

How many blades does the fan have?

Cheers

Greg Locock

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[pippogarruz]

the wheel has 22 blades. 11 per side (it's a double inlet).
The blades on the two sides are not alligned.

Filippo Garuti

 

[GregLocock]

You might want to steer clear of blade passing frequency as well, which admittedly should not be a problem with that number of blades

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[EricZhao]

Did you analysis your natural frequency at static state of at the rotating state. When the rotor is working, its natural frequency would increase slightly by the gyroscopic force. The mount of the increment dependents on both the rotating speed and the geometric shape of the rotor. So be very careful, I will say 30% higher than your 19Hz or 20% lower than 19Hz.

Also donot forget the second, third… natural frequency.

 

[franck]

Hi EricZhao and Filippo,

Actually, for a rocking mode acting on a rotating wheel, the gyroscopic theory predicts that the original mode (static) will split into two speed dependent branches. I have seen this effect on satellite reaction wheels as from wheel speed of 300 rpm but obviously this depends on the geometry of your system.

Best regards,

Franck

 

[EricZhao]

franck:

Thanks for your input. You are right. For positive whirl, the natural frequency increases with the rotational speed. For negative whirl, the natural frequency decreases with the rotational speed. But in most cases, only positive whirl is considered in rotor dynamics.